Photonic crystals are periodically structured electromagnetic media possessing photonic band gaps (“PBG”) for certain range of frequencies, the PBG being mainly a function of the spatial periodicity parameters of the crystal and the refractive indices of the base material(s)[1,15]. By controlling these physical aspects of the photonic crystals one can generate a wide range of PBGs as per the need of the required application. The application of PBGs range from small-scale optical interconnects/circuits for all-optical signal processing to very narrow wavelength light sources to narrow wavelength filters and optical switches. Hence, there has been an enormous interest in photonic crystal research in the last few years in order to exploit the photon manipulation possibility they offer.
Most photonic crystals, however, have specific properties that cannot be varied once the crystals are made. Hence it is highly desirable to have the PBG tunability in a single photonic crystal by various external effects as it would lead to other interesting photonic components such as optical switches, tunable filters and optical interconnects/circuits. This possibility to dynamically control the photonic properties of the crystals through various external influences has led to the fabrication and study of a wide variety of tunable photonic crystals. The ultimate aim is to maximize the optical frequency (wavelength) range over which tunability can be achieved and the response speed of the tunability.
One type of prior art tunable crystal relies on the reversible change of the physical attributes of the photonic crystal such as the lattice spacing while another relies on the tunability of the refractive indices of the crystal material through various external effects. The physical aspect change has been achieved either through mechanical[16], piezoelectric[17], electrical/magnetic[18] effects among others. Refractive index tunability has been proposed through the effects of electric field in liquid crystals[19] as well as the infiltration of liquid crystal and polymeric[20] components in periodic structures.
Other methods of achieving tunability such as the variation of free electron carriers through temperature variation in semiconductors[21] and alteration of electric permittivity or magnetic permeability through external magnetic and electric fields[22] have also been proposed.
The tuning of the PBG of photonic crystals based on the phenomenon of electrochromism (“EC”) has been proposed by the inventors in which a reversible optical property change can be induced in some transition metal oxide (“TMO”) thin films such as Tungsten trioxide (WO3) through the double injection of ions and electrons[5]. The ensuing change in the optical constants through the application of a small electric field and the said double insertion/extraction brings about the PBG tunability. This EC phenomenon is very practically amenable to the fabrication of small scale photonic crystal devices based on inverse opals of the TMOs.
Transition metal oxides (TMO) with their multiple oxidation state present a variety of interesting properties of which the reversible EC coloration under ion/electron injection has attracted lot of research attention lately due to its rich application potential in smart windows, display devices, optical modulators etc[3]. The bulk of the research on TMOs is directed towards the tungsten trioxide thin films which are found to be the best candidates for EC application.
Through our earlier work[5] carried out on Tungsten trioxide based inverse opal photonic crystals, we have amply demonstrated the possibility of PBG tunability under the lithium atom intercalation and the underlying EC phenomenon. A continuous tunability of two Bragg diffraction peaks in the hexagonal macroporous structure could be achieved. This earlier work served as a distinct tool to verify the potential for tunability as the inserted lithium atoms could not be extracted from the applied sample configuration to examine the reversible operation.
The present disclosure relates to the fabrication of a complete electrochromically tunable photonic crystal device to achieve both insertion and extraction of the ion/electron species, and to chromogenically tunable photonic crystal devices generally.